Field of Invention
The present invention relates to self-foaming hot melt adhesive compositions and methods of making and using the same. More particularly, the invention relates to self-foaming thermoplastic hot melt compositions, methods of manufacturing a self-foaming thermoplastic hot melt compositions, processing apparatus for activating self-foaming thermoplastic hot melt compositions and methods for adhesively bonding one or more substrates together using self-foaming thermoplastic hot melt compositions.
Brief Description of Related Art
Foamed adhesives have closed cell gas bubbles uniformly distributed throughout the matrix. There are numerous benefits to the use of foamed hot melt adhesives including reduced adhesive consumption for equivalent bond performance, longer adhesive open time, lower BTU content per equivalent volume of adhesive, decreased weight per bond leading to lower cost and lower energy consumption. Various foam hot melt compositions and methods of making same are described including, for example, in U.S. Pat. Nos. 4,200,207, 4,059,714, 4,059,466, 4,555,284 and in WO 2013/078446.
A known method to produce hot melt foam is to meter and mix an inert gas into the molten hot melt at elevated pressure, for example, 300 psi and above. The gas dissolves into the hot melt under pressure but creates foam when the molten material is dispensed from the pressurized dispensing equipment into standard atmospheric pressure. Unfortunately, this mechanical process does not produce consistent and uniform foam density.
U.S. Pat. No. 4,059,714 teaches use of a pump as described in U.S. Pat. No. 4,200,207. This pump has two stages wherein a gas is supplied to the second stage to be mixed with hot melt, pressurized, and ultimately produce foamed hot melt when dispensed into atmospheric pressure. In practice, it is known that cavitation occurs in the second stage of the pump, which causes it to wear rapidly. As the pump wears, the quality and density of the foam decreases, thus requiring frequent costly repair or replacement of the pump. This process requires a recirculating loop that returns undispensed hot melt to the second stage of the pump. Recirculation is achieved with heated return hoses which are expensive and cumbersome. Material being returned to the second stage of the pump contributes to the cause of non-uniform foam density.
U.S. Pat. No. 4,059,466 describes the benefits of foamed hot melt in detail. The claims of this patent define a method of bonding with thermoplastic adhesive by “heating solid thermoplastic adhesive and a blowing agent”, then pressurizing said material, heating it to the decomposition temperature of the blowing agent then dispensing the molten hot melt into standard atmospheric pressure wherein it expands into a closed cell foam. This process is not used in industry and has not gained commercial acceptance due to several technical obstacles, including:                a) The solid thermoplastic adhesive granulate and the blowing agent powder exists in extremely different particle sizes. When these components are mixed together the powdered blowing agent is randomly distributed throughout the hot melt granulate. Therefore, some portions of the hot melt have no blowing agent while other portions have very high concentrations of blowing agent. The non-uniform distribution of blowing agent causes extreme variability in foam density making it commercially unacceptable. Those portions of hot melt with a high concentration of blowing agent expands in such great volume that they cause the foam to collapse and also produce air gaps and voids in the extrudate. Similarly, portions of the hot melt with no blowing agent do not foam at all.        b) The blowing agent powder settles by gravity making direct contact with the heated tank floor and tank walls. Some of this blowing agent powder decomposes even before the granulate hot melt becomes molten. This further contributes to unacceptable foam uniformity in commercial use.        c) U.S. Pat. No. 4,059,466 describes a melt temperature of 250° F. for the hot melt blowing agent powder mix. This is an impractically low temperature to accommodate high speed automatic application of hot melt adhesives. At 250° F., the melt rate of most hot melts is below the demand rate of automated production lines.        d) Hot melt application equipment is generally accessible to production line personnel. It is common place for these individuals to change equipment, temperature and pressure settings. The blowing agent specified in this patent begins to decompose and generate gas at temperatures of 350° F. and above. As shown on FIG. 1, which is a publicly available graph showing the decomposition rate of azodicarbonamide (CELOGEN AZ-130) in dioctyl phalathate, the decomposition rate is a function of time-at-temperature. The method disclosed in this patent has no provision to monitor the accumulated thermal history of the molten hot melt blowing agent mix. Therefore, when the accumulated thermal history reaches the decomposition point, hot melt foam will be generated in a melt tank open to atmosphere. When foam expansion occurs in the melt tank, the foam overflows from the tank contaminating the surrounding area presenting dangerous burn hazards and incinerating electronic controls.        
U.S. Pat. No. 4,059,466 also describes a method of producing a hot melt foam by first heating solid hot melt particles and a blowing agent powder blend to a temperature at which the blend becomes molten, but below the decomposition temperature of the blowing agent (T-1), then pumping and pressurizing said molten composition through a heat source to increase its temperature to the decomposition temperature of the blowing agent (T-2), then dispensing said composition into atmospheric pressure upon which it expands into a hot melt foam. This patent specifies an adhesive application temperature of approximately 375° F., which is below the temperature needed to decompose 100% of the blowing agent. At 375° F., the amount of gas evolved will depend on the length of time the material is held at that temperature. Therefore, foam density will change when material consumption rates change.
Chemical blowing agents decompose to produce a gas at elevated temperatures. Decomposition rates are a function of time and temperature. As temperature increases, the length of time needed to activate and decompose the blowing agent decreases (see, e.g., FIG. 1).
FIG. 1 reveals that at temperatures of 383° F. and below, not all of the azodicarbonamide decomposes, even after 30 minutes at temperature. Unless temperatures of 392° F. and above are achieved, the amount of azodicarbonamide that decomposes at lower temperatures will vary. Therefore, the foam density resulting from the volume of gas produced depends upon the length of time the material is held at any given temperature below 392° F.
In automated hot melt applications, the amount of hot melt consumed per unit time constantly changes as production lines change speed or during idle time; for example, to change dispensing nozzles, to fix line jams, for lunch breaks, or any other situation that interrupts the rate of material consumption.
Because of this variability in hot melt consumption, any decomposition temperature below 392° F. will result in inconsistent and changeable blowing agent decomposition and the amount of gas evolved. This, in turn, results in variability in the density of the hot melt foam produced. Variable foam density is unacceptable in automated hot melt production lines because variable volume of adhesive deposited could cause bond failure, changeable set time (too fast or too slow), and variable bond surface area.
Also, at temperatures of 375° F. and above, hot melt are subject to thermal degradation, loss of physical properties and lower adhesive performance. In addition, at these elevated temperatures most hot melts have a viscosity that is too low to support formation of acceptable foam due to cell coalescence, cell breakage and foam shrinkage.